1. Field of the Invention
The invention, in general, relates to a method of, and to an apparatus for, measuring viscosity and, more particularly, to the viscometric measurement of minute quantities of fluid substances.
2. The Prior Art
The viscometric observation of the activity of hydrolytic enzymes, the speed of polymeric synthesis, the course of clotting or coagulation processes and of other biotechnologically important processes is well-known. However, measurements with conventionally used viscometers, such as, e.g. rotation viscometers or capillary viscometers, usually require relatively large quantities of fluid. As described by W. M. Kulicke in Flowing Behavior of Substances and Substance Mixtures (Hüthig Wepf. publ. Basel, Heidelberg, New York, 1986), samples of the fluids are removed from their environment and transferred to the measuring device.
Yet for many tasks in polymer chemistry, biochemistry and physiology there exists, particularly in connection with highly viscous fluids, a demand for a viscosity measuring probe capable of analyzing quantities in the range of micro liters, for instance, on micro titer panels, within short intervals and without noticeably affecting the volume of the sample or environment. Miniaturization is particularly important in the context of the development of implantable or semi-invasive viscometric affinity sensors. Such sensors are based on a combination of viscometric affinity essay and micro dialysis, and they make it possible continuously to measure the concentration of glucose in a physiological system; see, for instance, Ehwald, R., Ballerstädt, Dautzenberg in Anal. Biochem. 234, 1-8; 1996, and Beyer, P. U., Ballerstädt, R., Ehwald, R., Grocer. biotechnol. 13, 143-146, 1996, where the viscosity of a sensitive fluid within the dialysis fiber serves as the measuring parameter. In the disclosed viscometric glucose sensor, the sensitive fluid is a concentrated solution of dextran molecules cross-linked by affinity bonding with the tetravalent glucose-binding lectin concanavalin A (ConA). Glucose diffusing from the exterior into the dialysis fiber lumen displaces the terminal glucose units of the cross-linked dextran molecules from their affinity bonding with the lectin and decreases the viscosity of the sensitive fluid as a function of the concentration; see Beyer, P. U., Ballerstädt, R., Ehwald, R., Lebensm. Biotechnol. 13, 143-146, 1996.
An implantable micro sensor for the subcutaneous determination of glucose based on affinity viscometry is disclosed by German patent specification 195 01 159 A1. In that micro sensor, a hollow fiber for the dialysis and a measuring system are hydraulically connected to an enclosed completely fluid-filled flow system. The measuring system is provided with a micro engine for moving the sensitive fluid and with a pressure, volume or flow sensitive transducer.
A further viscometric affinity sensor is known from German patent specification 197 14 087 A1, in which the diffusion of analyte and the measurement of viscosity are sequentially carried out at different locations. The sensor requires the sensitive fluid to flow through a hollow fiber segment serving as a dialysis chamber at a viscosity which is strongly dependent on the analyte. The viscosity of the sensitive fluid exiting from the segment after modification by the diffusible analyte constitutes a measure of the concentration of the analyte in the vicinity of the segment.
The processes of measuring viscosity changes in a dialysis hollow fiber segment are based upon either measuring the flow resistance of the sensitive fluid in the hollow fiber segment itself (German patent specification 195 01 159 A1) or in a capillary positioned downstream from the hollow fiber segment (German patent specification 197 14 087 A1). Since these processes require a pumping device and a viscosity measuring device located outside of the dialysis hollow fiber, the sensitive fluid in the dialysis fiber segment is, in the known viscometric affinity sensors, hydraulically connected to a fluid volume remote from the segment. In such sensors, it is necessary by special structural measures to prevent, or at least limit, the diffusive exchange with a dead volume of the sensitive fluid in order to avoid undefined delays in signal generation. In accordance with German patent specification 197 14 087 A1, this is accomplished by a constant resupply of fresh sensitive fluid from a reservoir. It is, however, unlikely that such a system may be realized in an implanted sensor.
Furthermore, a sensor for measuring viscosity and density is known from German patent specification 198 04 326 A1. The sensor is provided with a flexible tongue made from silicon nitride, silicon oxide, a metal or from a compound material, and with an oscillator mechanically coupled to the flexible tongue for imparting oscillations to it. The measuring medium is analyzed by evaluating the oscillations. Such and similar sensors based upon the attenuation of oscillations (e.g. German Patent specification 198 06 905) cannot, however, be applied to measuring the viscosity of small quantities of highly viscous fluids in which no oscillations can be generated because of the extremely high attenuation.
Other known or obvious possibilities of measuring the viscosity in very small fluid volumes, such as, for instance, an optical analysis of Brownian particle movement, electrophoresis or dielectrophoresis, may not easily be applied to highly viscous polymeric solutions of an undefined electrolyte composition.
Hitherto, no apparatus adaptable to miniaturization and cost efficient fabrication has become known which is suitable for taking precise measurements of high viscosities in a very small and static measuring zone, such as, for example, a dialysis chamber, and for converting such measurements into electrical signals.